The world nuclear powers plan to convert nuclear fuel used in the manufacture of weapons to commercial grade UF.sub.6 to eliminate weapon grade material and provide energy for peaceful purposes. The fuel will contain other isotopes in small quantities, such as .sup.99 Tc, .sup.236 U, .sup.234 U, etc., due to previous fuel reprocessing, reactor operation, and/or contamination of the enrichment cascades. Consequently, new analytical procedures and experimental techniques must be developed for commercial production of this new type of fuel. An important and integral part of this process is to accurately assay uranium-containing materials to determine levels of technetium as well as other isotopes.
The isotope technetium-99 (.sup.99 Tc) is a fission product that is present in UF.sub.6 gas as an impurity, due to its accumulation in the enrichment cascades. Enriched commercial grade UF.sub.6 (ECGU) as defined in the ASTM Specification C-996 requires the level of technetium-99 to be less than 0.2 micrograms/gram of .sup.235 U. Technetium-99 originates from spent nuclear fuel that has been reprocessed into UF.sub.6 gas. When reprocessed UF.sub.6 is fed to the enrichment cascades, technetium-99 accumulates in the upper states. It then becomes an impurity, when non-reprocessed, natural UF.sub.6 is fed to the cascades for enrichment purposes.
Prior proposals have not provided rapid and accurate analysis of technetium-99. For example, Morita et al. ("Determination of .sup.99 Tc in Environmental Samples by Inductively Coupled Plasma Mass Spectrometry", Radiochimica Acta, pp. 63-67, 1993) described an analytical technique using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to determine Tc-99 in environmental samples. These samples do not contain uranium as an impurity, but ruthenium-99 (.sup.99 Ru) instead. This isotope interferes with the determination of .sup.99 Tc. An involved separation procedure was developed to eliminate the large amount of matrix elements present in these environmental samples. Organic matrix compounds were removed by ashing at 450.degree. C. The inorganic matrix elements were eliminated by coprecipitation with iron hydroxide followed by extraction with TBP-xylene. This was followed by back extraction with NaOH. Tc and ruthenium were then separated from solution by anion exchange. Elution of Tc and ruthenium from the anion exchange resin were accomplished with 8M NHO.sub.3. In this process, .sup.106 Ru could be efficiently separated from technetium by extraction of .sup.99 Tc With cyclohexanone. Back extraction with carbon tetrachloride and water provided an aqueous solution for ICP-MS analysis. Tc-95 m was used as a tracer to determine the chemical recovery. ICP-MS had a lower detection limit and shorter analytical measurement time when compared with conventional determination methods, which use beta counting.
Alonso, Sena, and Koch ("Determination of .sup.99 Tc in Nuclear Samples by Inductively Coupled Plasma Mass Spectrometry", Journal of Analytical Atomic Spectrometry, Vol. 9, November 1994) reported that in measuring .sup.99 Tc with ICP-MS, matrix effects from uranium were not observed at concentrations below 500 .mu.gm/ml. However, Makinson ("The Comparison of Sample Preparation Techniques for Determination of Technetium-99 in Pure Uranium Compounds and Subsequent Analysis by Inductively Coupled Plasma-Mass Spectrometry", a Paper given at the ASTM/DOE conference, October 1994, Gatlinburg, Tenn.) found at 2000 ppm(v), the technetium signal was suppressed 2 to 3 times, while at 5000 ppm(v) the suppression was 10 times that obtained for the technetium solution without uranium. At these uranium concentrations, the ECGU limit of 0.2 .mu.gm/gm .sup.235 U could not be measured because of technetium peak suppression by uranium.
Our measurements using the ICP-MS showed the suppression of the uranium signal was at least this order of magnitude. Thus, a new analytical procedure was required to determine technetium within a high uranium matrix, such as an ammonium diuranate (ADU) precipitate, uranyl fluoride solution, UO.sub.2 powder, and the like. Such a procedure would allow one to monitor the aqueous streams of the ADU process, including the intermediate processing steps and final product.